The local production of 1,25(OH)2D3 promotes osteoblast and osteocyte maturation

Maintenance of an adequate vitamin D status, as indicated by the level of circulating 25-hydroxyvitamin D (25(OH)D), is associated with higher bone mass and decreased risk of fracture. However, the molecular actions of vitamin D hormone (1,25(OH)(2)D-3) in bone are complex, and include stimulation of osteoclastogenesis via RANK-ligand up-regulation, as well as the inhibition of mineralisation. We hypothesise that these divergent data may be reconciled by autocrine actions of 1,25(OH)(2)D-3 which effect skeletal maintenance, as opposed to endocrine 1,25(OH)(2)D-3 which acts to maintain serum calcium homeostasis. We have previously described local metabolism of 1,25(OH)(2)D-3 within osteoblasts, with effects on gene expression and cell function. The aim of the current study was to investigate potential autocrine actions of 1,25(OH)(2)D-3 within cells that exhibit osteocyte-like properties. Late osteoblastic MLO-A5 cells were cultured in the presence of 25(OH)D for 9 days with gene expression analysed pre- and post-mineralisation. Gene expression analysis revealed maturation within this time frame to an osteocyte-like stage, evidenced by increased Dmp1 and Phex mRNA expression. Expression of Cyp27b1 in 25(OH)D treated MLO-A5 cells was associated with elevated media levels of 1,25(OH)(2)D-3 (p < 0.05), induction of Cyp24a1 (p < 0.001) and elevated ratios of Opg:Rankl mRNA (p < 0.01). Chronic 25(OH)D exposure also increased osteocalcin mRNA in MLO-A5 cells, which contrasted with the dose-dependent inhibition of osteocalcin mRNA observed with acute treatment in MLO-Y4 cells (p < 0.01). Treatment of MLO-Y4 cells with 25(OH)D also inhibited Phex mRNA expression (p < 0.05), whilst Enpp1 gene expression was induced (p < 0.01). Overall, the current study demonstrates that osteocyte-like cells convert physiological levels of 25(OH)D to 1,25(OH)(2)D-3, with changes in gene expression that are consistent with increased osteocyte maturation. Although the physiological role of local metabolism of 1,25(OH)(2)D-3 within osteocytes requires further investigation, the abundance and diverse functions of this cell type within bone underscore its potential importance. This article is part of a Special Issue entitled '16th Vitamin D Workshop'. (C) 2013 Elsevier Ltd. All rights reserved.